The present invention generally relates to a storage system and its control method, and, for instance, can be suitably applied to a storage system adopting a flash memory as its memory device.
Conventionally, a semiconductor memory or a hard disk drive is used as the memory device in a storage system. A semiconductor memory boasts such benefits as fast access speed, compact size, low power consumption and high reliability, but also has a drawback in that the cost per unit bit is expensive. Meanwhile, a hard disk drive has drawbacks in comparison to a semiconductor memory such as slow access speed, bulky size, high power consumption and low reliability, but is advantageous in that the cost per unit bit is dramatically cheaper than a semiconductor memory.
Thus, in recent years, hard disk drives have become mainstream as the memory device in a storage system, and technological innovation relating to hard disk drives has accelerated. As a result, the storage capacity per unit area in a hard disk drive has increased by leaps and bounds.
In addition, reliability, which was the weak point of hard disk drives, has improved due to the application of RAID (Redundant Array of Inexpensive Disks) technology, and in recent years a large-scale storage system with a capacity of several TB has appeared in the market.
Meanwhile, in recent years, a flash memory as a semiconductor device enabling the free rewriting of data and which retains data even when the power is turned off is being widely used as a memory device. Pursuant to the diffusion of this flash memory, the cost per unit bit of flash memories is coming down.
Under the foregoing circumstances, it is anticipated that a storage apparatus equipped with a flash memory as the memory device will hit the streets in the future. By adopting the foregoing configuration, it is assumed that a storage apparatus with fast access speed and low power consumption can be realized.
Incidentally, Japanese Patent Laid-Open Publication No. 2004-21811 proposes a storage that consolidates a flash memory and a hard disk drive.